thunderbolt: Compile on x86 only

So far Thunderbolt is (unfortunately) an Intel proprietary technology
that is only available on x86, so compiling on other arches is pointless
except for testing purposes.  Amend Kconfig accordingly.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andreas Noever <andreas.noever@gmail.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/7dfda728d3ee8a33c80c49b224da7359c6015eea.1479456179.git.lukas@wunner.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>